The invention relates to a path control apparatus for a computer-directed control of a numerically controlled machine tool in general and more particularly to an improved apparatus of this type.
Path control apparatus for numerically controlled machine tools with controlled drives in the feed axes, wherein a computer determines the trajectory to be traversed from input data are known. Such path control apparatus is described in SIEMENS-Zeitschrift 1966, pages 67-72.
FIG. 1 shows the structure of a known computer directed path control apparatus for a numerically controlled machine tool 9. The information, which is stored, for instance, on perforated tape or magnetic tape, is fed into a reference input computer 2 by a reader 1 during a programmed machining cycle. The reference input computer 2 decodes the information read in and determines therefrom the three dimensional trajectory of the path control. However, the variables determined by the reference input computer 2 are not suitable for setting the desired values for the position control loops directly. Instead, they must be interpolated in the controlled feed axes by an interpolator 3 which follows the reference input computer 2. The interpolator 3 determines reference values for the position control loops assigned to the individual feed axis drives. The position reference values determined by the interpolator 3 are compared in respective comparators 4 with the actual position values of the corresponding feed axes. The respective control difference drives the position controller 5 of the associated feed axis. The position controller 5 generates the reference input for a speed control loop comprising a speed controller 7 and the drive system 8 of the controlled feed axis. The speed reference values determined by the position control 5 are compared in a comparator 6 with the actual speed values of the drive system 8. The speed difference drives the speed controller 7. The latter in turn drives the drive system 8.
Computer directed path control systems are also known in which the reference input computer itself performs a preliminary interpolation of the three dimensional trajectory, while an interpolator following the computer for each feed axis performs a post-interpolation. In the known computer directed path control systems of numerically controlled machine tools, however, the reference input computer is followed in every case by one interpolator per feed axis, which is realized as a separate circuit, and which is followed in turn by a position controller, which is likewise realized as a separate circuit.